1. Field of the Invention
The present invention relates to a valve timing control apparatus for controlling valve timing of a valve that is opened or closed by a camshaft through torque transmitted from a crankshaft in an internal combustion engine.
2. Description of Related Art
Conventionally, a fluid-drive valve timing control apparatus is known, and has a housing rotatable with a crankshaft and a vane rotor rotatable with a camshaft. A valve timing is controlled by working fluid supplied from a supply source, synchronized with a rotation of an engine. The control apparatus controls working fluid to flow in or out of operation chambers partitioned by vanes of the vane rotor in a rotating direction inside of the housing, thereby changing a rotation phase of the vane rotor relative to the housing.
JP-A-2003-314311 describes a fluid-drive valve timing control apparatus having a lock mechanism to lock the rotation phase, in which an unlock chamber communicating with a predetermined operation chamber is provided. If a pressure of working fluid flowing into the unlock chamber from the predetermined operation chamber is raised, the rotation phase is unlocked.
If a pressure of working fluid supplied from the supply source is low immediately after an engine is activated, the unlock of the rotation phase may become late. In this case, the rotation phase is restricted from having a smooth change in a normal control in which working fluid is controlled relative to each operation chamber. Therefore, the normal control is set to be allowed after the unlock of the rotation phase is detected in JP-A-2003-314311. However, the execution of the normal control becomes late because the detection of the unlock has to be waited. In this case, gas mileage and exhaust gas characteristics may become worse.